Hedgehog Spin-vortex Crystal Antiferromagnetic Quantum Criticality in CaK(Fe1-xNix)4As4 Revealed by NMR

Abstract

Two ordering states, antiferromagnetism and nematicity, have been observed in most iron-based superconductors (SCs). In contrast to those SCs, the newly discovered SC CaK(Fe${}_{1-x}$Ni${}_x$)${}_4$As${}_4$ exhibits an antiferromagnetic (AFM) state, called hedgehog spin-vortex crystal structure, without nematic order, providing the opportunity for the investigation into the relationship between spin fluctuations and SC without any effects of nematic fluctuations. Our ${}^{75}$As nuclear magnetic resonance studies on CaK(Fe${}_{1-x}$Ni${}_x$)${}_4$As${}_4$ (0$\le x\le$ 0.049) revealed that CaKFe${}_4$As${}_4$ is located close to a hidden hedgehog SVC AFM quantum-critical point (QCP). The magnetic QCP without nematicity in CaK(Fe${}_{1-x}$Ni${}_x$)${}_4$As${}_4$ highlights the close connection of spin fluctuations and superconductivity in iron-based SCs. The advantage of stoichiometric composition also makes CaKFe${}_4$As${}_4$ an ideal platform for further detailed investigation of the relationship between magnetic QCP and superconductivity in iron-based SCs without disorder effects.

6 pages, 5 figures, accepted for publication in Phys. Rev. Lett